A thin film transistor (TFT) has been widely utilized as a switching element and a driving element in an electronic device. Specifically, due to the possibility of formation of the thin film transistors on a glass substrate or a plastic substrate, they are commonly employed in the flat panel display field, such as a liquid crystal display (LCD), an organic light emitting diode (OLED), and etc.
The oxide semiconductor has higher electron mobility (the mobility of the oxide semiconductor is >10 cm2/Vs and the mobility of the amorphous silicon (a-Si) is merely 0.5˜0.8 cm2/Vs) and has simpler manufacture process in comparison with the Low Temperature Poly-silicon (LTPS) and higher compatibility in comparison with the amorphous silicon process. Therefore, it can be applicable to the skill fields of Liquid Crystal Display, Organic Light Emitting Display, Flexible Display and etc. Because it fits the new generation production lines and the possible applications in displays with Large, Middle and Small sizes. The oxide semiconductor is hot to the research field of the present industry because the great opportunity of application development.
The oxide semiconductor has advantages of higher electron mobility and simpler manufacture process. Nevertheless, some drawbacks exist. As illustrations, the stability is bad and big influence with temperature and humidity exists. The electric property of the oxide semiconductor drifts with time. The oxide semiconductor requires higher demands for process conditions, such as the film formation rate, the atmosphere and the temperature of the process, control of the temperature and etc. Besides, higher demands are further required for the isolation layer, the contact between the oxide semiconducting layer and the isolation layer, the contact between the oxide semiconducting layer and the metal layer. Restrict demanding the oxide semiconducting layer itself is not enough as regarding an oxide thin film transistor.
Please refer to FIG. 1, which is a sectional view diagram of a structure of a bottom gate type oxide thin film transistor according to prior art. The manufacture method of the structure of the oxide thin film transistor mainly comprises: step 1, providing a substrate (100); step 2, forming a gate (200) on the substrate (100); step 3, covering the gate (200) with a gate isolated layer (300); step 4, forming an oxide semiconducting layer (400) on the gate isolated layer (300); step 5, forming an etching stopper layer (500) on the oxide semiconducting layer (400); step 6, respectively forming two vias (510, 530) in the etching stopper layer (500) to expose the oxide semiconducting layer (400); step 7, forming a source (610) and a drain (630) on the etching stopper layer (500), and the source (610) fills one via (510) and connects with the oxide semiconducting layer (400), the drain (630) fills the other via (530) and connects with the oxide semiconducting layer (400); step 8, forming a protective layer (700) on the source (610) and the drain (630) for covering the source (610) and the drain (630).
In the aforementioned manufacture method of the structure of the oxide thin film transistor, the etching stopper layer (500) of step 5 is generally formed by a SiOx film layer which is manufactured with TEOS+O2 or SiH4+N2O chemical vapor deposition. However, the plasma will affect the surface property of the oxide semiconducting layer (400) when the film of the etching stopper layer (500) is formed. For instance, SiH4+N2O contains hydrogen which may combines with the oxygen in the oxide semiconducting layer (400) and leads into increase of the oxygen defects. As a result, the threshold voltage Vth tends to be negative. The oxygen in TEOS+O2 leads into decrease of the oxygen defects in the oxide semiconducting layer (400) and reduces the conductivity of the trench. Therefore, what contacts with the source (610) and the drain (630) is the damaged surface of the oxide semiconducting layer (400). Ultimately, the electrical property of the oxide thin film transistor is affected. Please refer to FIG. 2, which is a curve diagram of the electrical property of an oxide thin film transistor according to prior art. As shown in the figure, the threshold voltage Vth=−5V and subthreshold swing S.S=0.45 when the voltage of the drain Vd=10V. The electrical property of the oxide thin film transistor can be worse.